Conventionally, portable telephones have been widely spread, and TDMA (time division multiple access) and FDMA (frequency division multiple access) were used for access methods for portable telephones. In these days, CDMA (code division multiple access) is being adopted instead of TDMA and FDMA because of various merits, such as high efficiency at usage of frequency band, facility of change of transmission rate, and preservation from eavesdropping.
However, CDMA according to prior art is prepared mainly for voice transmission and therefore is not suitable for data communication. In recent years, as the development of multimedia, not only voice but also various kinds of data that can be processed in computers and so on should be transmitted. Therefore, communication access between mobile stations and network should be suitable for transmitting various types of data in the near future.